Implantable medical devices (IMD) and associated implantable medical leads provide functions such as stimulation of muscle or neurological tissue and/or sensing of physiological occurrences within the body of a patient. Typically, the IMD is installed in a subcutaneous location that is accommodating and relatively accessible for implantation. For instance, to provide stimulation near the spine or pelvis, the IMD may be installed in a pocket located within the abdomen or upper buttocks region of the patient. For stimulation of the brain or heart, the IMD may be installed in a pocket located in the pectoral region. Stimulation leads are routed to the appropriate stimulation site and terminate at the pocket where they may be connected to the IMD.
After installation, movement of the IMD within the pocket, and rotational movement in particular, is a concern. An external housing of the IMD is typically a smooth metal such as titanium that does not resist movement relative to the tissue in the pocket. Such movement may result from the normal daily activities of the patient. Movement may also occur due to the patient manipulating the IMD position by grasping the IMD through the skin. The movement may cause various problems.
The implantable medical leads have electrical conductors within them. Typically, the lead is installed so that slack in the lead is removed. For instance, excess length of the lead may be wrapped about the IMD within the pocket to reduce the amount of slack. Excess movement of the IMD, and particularly rotation or flipping of the IMD, may stress the electrical conductors within the leads and cause breakage of the conductors. A surgical procedure to replace the broken lead may be necessary.
Another issue for IMDs that utilize unipolar stimulation is that the back side of a metal external housing of the IMD is coated with an insulator so that the front side of the metal can acts as one of the electrodes. Maintaining this housing electrode closest to the surface of the patient's body reduces the likelihood of uncomfortable sensations occurring within the tissue beneath the IMD. Should the IMD be flipped such that the uninsulated side of the metal can faces the tissue beneath the IMD, then the uncomfortable sensations become more likely.
Another issue occurs for IMDs that utilize a rechargeable battery and associated recharging circuitry. For these IMDs, the side of the IMD where a recharging coil is located is positioned closest to the surface of the patient's body so that energy transfer through an inductive coupling with an external coil can occur. Should the IMD be flipped such that the opposite side of the IMD becomes the closest to the surface of the patient's body, the inductive coupling is adversely affected and the IMD may not be properly recharged. If the IMD cannot be externally flipped back to the proper orientation, then a surgical procedure may be necessary to flip the IMD back to the original orientation so that recharging can commence.